1. Field of the Invention
The present invention relates to an electrode for a fuel cell and a method for manufacturing the same.
2. Description of the Related Art
A solid polymer electrolyte fuel cell is composed of an electrolyte of an ion exchange membrane such as a perfluorosulfonic acid membrane and electrodes of an anode and a cathode bonded to both surfaces thereof. The electrolyte fuel cell generates a power under an electrochemical reaction by supplying a reducing agent (e.g. hydrogen, methanol, hydrazine, etc.) to the anode and an oxidizing agent (e.g. air, oxygen, etc.) to the cathode. The electrochemical reaction occurring in each electrode using oxygen as the oxidizing agent and hydrogen as the reducing agent is as follows.
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
Entire reaction: H2+1/2O2xe2x86x92H2O
As understood from this reaction, the reaction in each electrode proceeds only in a three-phase boundary where a gas (hydrogen or oxygen) that is an active material, a proton (H+) and an electron (exe2x88x92) are simultaneously transferred.
An example of the electrode having such a function is a composite electrode of a solid polymer electrolyte-catalyst containing carbon particles and catalyst material as well as a solid polymer electrolyte. The macroscopic state of this electrode is shown conceptually in FIG. 2. In FIG. 2, reference numeral 21 denotes carbon particle, 22 a solid polymer electrolyte, 23 one of pores, and 24 an ion exchange membrane. As seen from FIG. 2, the composite electrode is a porous electrode in which carbon particles 21 supporting the catalyst material and solid polymer electrolyte 22 are mixed with each other so that they are distributed three-dimensionally, and plural pores are formed. In this composite electrode, the carbon supporting the catalyst constitutes an electron conductive channel, the solid polymer electrolyte constitutes a proton conductive passage and the pore constitutes a channel of supplying/discharging oxygen or hydrogen and water which is a product. Within the electrode, these three channels are distributed three-dimensionally so that an infinite number of three-phase boundaries capable of transferring gas, protons (H+) and electrons (exe2x88x92) simultaneously are formed to provide a field of the electrode reaction.
Conventionally, the electrode having such a structure has been manufactured by a method comprising the steps of preparing a paste composed of a catalyst supported on carbon particles (in which platinum group metal particles such as platinum particles are supported on carbon particle with highly dispersion) and a PTFE (polytetrafluoroetylene) particle dispersed solution, dispersing the paste on the polymer film or a carbon electrode substrate of an electro-conductive porous material to make a film (generally having a thickness of 3-30 xcexcm), heating/drying the film, and applying a solid polymer electrolyte solution onto the film so that the film is impregnated with the solution. The electrode has been also manufactured by the method comprising a steps of preparing a paste composed of the above catalyst supported on carbon particles, PTFE particles and a solid polymer electrolyte solution, dispersing the paste on the polymer film or the carbon electrode substrate of electro-conductive porous material to make a film (generally having a thickness of 3-30 xcexcm), and thereafter heating/drying the film. Incidentally, the solid polymer electrolyte solution was prepared by dissolving the material having the same composition as the ion exchange membrane described above in alcohol to provide a solution. The PTFE particles dispersed solution was prepared as a solution dispersed with PTFE particles having a particle diameter of about 0.23 xcexcm.
The solid polymer electrolyte fuel cell, which has advantages of capable of being actuated at room temperature and being compact and light as well as having a high power, has been developed for use in an application of an electric vehicle. Such a type fuel cell generally uses a gaseous fuel such as hydrogen as a reducing agent, or otherwise a liquid fuel such as methanol, hydrazine, etc.
The case of using hydrogen as the reducing agent includes systems of storing hydrogen in a highly compressed bomb, and storing hydrocarbon fuel such as methanol or natural gas as a raw material and reforming it into hydrogen for use by a reforming device. The latter system is predominant in view of the total cost and official infrastructure for circulation. The reforming reaction using methanol is as follows.
CH3OH+H2Oxe2x86x923H2+CO2+(CO) 
The system using this reaction has a disadvantage that a very small quantity of CO created as well as CO2 poisons the catalyst material such as platinum for the anode of the fuel cell and hence reduces the power.
The fuel cell which directly uses methanol of the liquid fuel as the reducing agent is called a direct methanol fuel cell. This fuel cell has advantages that it is easy to deal with since the liquid fuel is used in the low temperature directly without using the reforming device, and it makes the entire system simple and compact because of unnecessity of the reforming device. However, this fuel cell has a disadvantage that it provides a higher overvoltage due to oxidation of the fuel than the fuel using the gaseous fuel since methanol is oxidized at a low speed, and a large amount of noble metallic catalyst is required in the anode, thus increasing the production cost of the fuel cell.
Nowadays, these problems have been improved greatly by compositely using plural metals as the catalyst.
For example, as a catalyst having a CO tolerance characteristic, an alloy catalyst of Ptxe2x80x94Ru, Ptxe2x80x94Sn, Ptxe2x80x94Pd, Ptxe2x80x94Mn or Ptxe2x80x94Co has been proposed. As a catalyst which is active for the electrochemical oxidation reaction of the liquid fuel such as methanol, an alloy catalyst of Ptxe2x80x94Ru, Snxe2x80x94Ir, Ruxe2x80x94Ir, Ptxe2x80x94Au or Pdxe2x80x94Ag has been proposed. The reason why activity of the catalyst in the form of an alloy is improved can be understood by some explanations, for example, alloying of platinum shortens the distance among platinum atoms, and the catalyst which is greatly meshed to have a highly active surface is solved away from the secondary metal (e.g. Ru, Sn, Pb, Rh).
The technique of alloying the catalyst has been also applied to the cathode catalyst. For example, it has been reported that the catalyst of Ptxe2x80x94Fe or Ptxe2x80x94Ni exhibits a higher activity for the reduction of oxygen than the catalyst of only platinum (see Masahiro Watanabe, the 38-th Battery Symposium in Japan NO.1I13, P29 (1997)).
A carbon supporting such an alloy catalyst can be acquired by impregnating carbon particles with two or more kinds of metal elements and reducing them. For example, a Ruxe2x80x94Ir alloy supported on carbon can be prepared by impregnating the carbon particles with a mixed water or alcohol solution of mixture of Ru and Ir compounds, drying them and thereafter reducing them by a hydrogen gas. In this case, the carbon particles are directly given the catalyst of mixed Ru and Ir in an atom level.
A Ptxe2x80x94Ru alloy supported on carbon can be prepared by impregnating the carbon particles with a water or alcohol solution of a platinum compound, drying them and thereafter reducing them by a hydrogen gas to provide a platinum supported on carbon, and further impregnating the carbon particles with a water or alcohol solution of a Ru compound, drying them and thereafter reducing them by the hydrogen gas. In this case, the carbon particle supports platinum fine particle covered with the layer of Ru. When the carbon particles are treated by hydrogen at a high temperature (500xc2x0 C.), the surface is deformed from the Ru layer into the Pt layer.
As described above, attempts of giving the activity which cannot be acquired using the catalyst consisting of a sole metal, by alloying two or kinds of metals have attained a preferable result when the carbon particles support a large amount of catalyst. However, when the amount of catalyst supported on carbon is lowered, the effect of alloying is not remarkable.
The reason therefor is as follows. When it is intended that the carbon particles having a very large surface area per unit weight are simultaneously impregnated into solution of starting catalyst material compounds consisting of two or more kinds of metals compounds and deposited, particles of these metal will be separated so that they will be dispersed solely with high dispersion, respectively, and hence not coagulated sufficiently. Thus, the degree of alloying will be lowered greatly. This will attenuate the CO tolerance characteristic and the activity in the electrochemical reaction of methanol.
From the aspect that the carbon which is a support of the catalyst makes an electron conductive channel, the solid catalyst makes a proton conductive passage and the pore makes a supply/discharge channel of oxygen or hydrogen and water which is a product, a conventional electrode for a fuel cell has been manufactured in the hope of that the catalyst material supported on the carbon particle happened to be located at the three-phase boundary formed in each channel. Such a manufacturing method required a large amount of catalyst. As regards the electrode manufactured by the method described above, it was reported that the utilization of catalyst supported on the carbon was as low as 10% (see Edson A. Ticianelli J. Electroanal. Chem., 251, 275 (1998)).
Namely, taking into consideration that the catalyst is uniformly dispersed at on the surface of the carbon particle, the catalyst at a low rate of 10% contributes to reaction on the surface of the carbon particle having an excessively large surface area. Nevertheless, conventionally, the catalyst was used in such a manner that it is supported on the entire surface of the carbon particle.
The inventors of this invention conceived that if two or more kinds of metals is mainly carried on the three-phase boundary sites relating to the reaction, even with a small amount of catalyst, the metal are not dispersed and hence alloying rate is not also lowered. Therefore, the inventors first investigated the electrode reaction microscopically in order to find the area where the catalyst acts effectively.
As understood from the reports by H. L. Yeager et al (J. Electrochem. Soc., 128, 1880 (1981)) and Ogumi et al (J. Electrochem. Soc., 132, 2601 (1985)), polymer electrolyte consists of proton conductive passage called cluster and hydrophobic backbone. The gas (hydrogen or oxygen) that is needed for electrochemical reaction and water that is a product for the cathode as well as protons translate through a proton conductive passage called a cluster that incorporate the hydrophilic ion-exchange functional group and its counter ion with water, and the hydrophobic backbone does not constitute a moving passage of gas, water and proton.
Therefore, the inventors conceived that the three-phase boundary where the reaction of a fuel cell electrode proceeds is present only on the surface of the carbon particle, constituting the electron conductive channel, in contact with the proton conductive passage called a cluster of the solid polymer electrolyte. Thus, the inventors found that it is necessary to examine the position and distribution of the catalyst material for the proton conductive passage in such a solid polymer electrolyte.
FIG. 3 is a conceptual view of the state of the surface of the carbon particle in contact with a solid polymer electrolyte in a conventional electrode. In FIG. 3, reference numeral 31 denotes a carbon particle, 32 a proton conductive passage, 33 a hydrophobic backbone, and 34, 35 catalyst material. In the conventional electrode, the surface of the carbon particle 31 is covered with the solid polymer electrolyte consisting of the proton conductive passage 32 and hydrophobic backbone 33 and supports the catalyst material 34, 35. However, the catalyst material 35 acts effectively since it is located on the carbon surface in contact with the proton conductive passage whereas the catalyst material 34 does not act since it is located on carbon surface in contact with the hydrophobic backbone 33.
It is an object of the present invention to improve the structure of a microscopic three-phase boundary of an electrode to enhance the catalyst utilization, thereby facilitating alloying of the catalyst.
The present invention has been accomplished on the basis of new findings that a three-phase boundary is located on the surface of the carbon particle, contributing to electron conduction, in contact with a proton conductive passage of a solid polymer electrolyte and a carbon particle contributing to electron conduction, and noting that the carbon particle shows catalytic activity for the reduction reaction of the above compound, and by ion exchange reaction between the compound and polymer electrolyte, a starting compound of catalyst can be preferentially adsorbed on the proton conductive passage of the solid polymer electrolyte.
According to the present invention, in an electrode for a fuel cell containing carbon particles and catalyst material as well as a solid polymer electrolyte, the catalyst material is mainly loaded in proton conductive passages of the solid polymer electrolyte and on the surface of carbon particles in contact with the proton conductive passages.
Preferably, in the electrode for a fuel cell containing carbon particles and catalyst material as well as a solid polymer electrolyte, the catalyst material is mainly loaded on the surface of carbon particles in contact with the proton conductive passages of the solid polymer electrolyte.
The expression of xe2x80x9cmainly loadedxe2x80x9d means that the amount of the catalyst material, loaded in proton conductive passages of the solid polymer electrolyte and on the surface of carbon particles in contact with the proton conductive passages, exceeds 50 wt % of the entire amount of catalyst, preferably the amount of the catalyst metal loaded on the surface of carbon particles which abut on the proton conductive passages of the solid polymer electrolyte exceeds 50 wt %.
In the electrode for a fuel cell according to the present invention, the catalyst material contains two or more kinds of metal elements, and includes two or more layers containing different metal elements.
A method of manufacturing an electrode for a fuel cell according to the present invention, comprises: a first step of adsorbing a starting compound of catalyst into a solid polymer electrolyte in a mixture containing solid polymer electrolyte and carbon particles, and a second step of chemically reducing starting compound of catalyst in the mixture obtained by the first step, wherein the starting compound of catalyst contains two or more metal elements.
Alternatively, a method of manufacturing an electrode for a fuel cell according to the present invention, comprises repeating twice or more an adsorption/reduction process including: a first step of adsorbing a starting compound of catalyst into a solid polymer electrolyte in a mixture containing solid polymer electrolyte and carbon particles, and a second step of chemically reducing the starting compound of catalyst in the mixture obtained by the first step, wherein the adsorption/reduction process of the starting compound of catalyst (a) is performed once or more, and the adsorption/reduction process of a starting compound of catalyst (b) which containing a metallic element not containing the starting compound of catalyst (a) is performed once or more.